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Follow the reluctant adventures in the life of a Welsh astrophysicist sent around the world for some reason, wherein I photograph potatoes and destroy galaxies in the name of science. And don't forget about my website, www.rhysy.net

Sunday, 17 March 2019

Are you one of the tiny handful of people who only follow me on this blog ?

Have you been wondering where I've been and why posts are even fewer lately than they normally are ?

Have you been secretly dreading that I must be preparing some absolute monster of a post that's gonna need a well-trained team of six stout mountaineers and a harpoon just to even read the abstract ?

Worry no more ! The reason has simply been that Google are a bunch of bastards who decided to cancel one of their greatest products : Google Plus. I shall save a paean on that for another day. Suffice to say that with the final shutdown approaching rapidly (2nd April), I did not particularly want to just watch my thousands of posts vanish into oblivion. Instead, using the Google Plus Exporter tool, I've been saving my assorted ravings and archiving into them into not one but two shiny new blogs.

Physicists of the Caribbean has evolved steadily over the years from wanton silliness to scientific analysis to philosophical ramblings. Posts here have also tended to spiral out of control in terms of length. See, what Google Plus let me do, apart from reaching an audience of some quite wonderful people, was jot down quick initial impressions without feeling obliged to explore anything in excessive detail. I've found that to be an incredibly useful experience and I don't want to lose it. Nor do I want to start doing the same thing on other social media only to potentially lose it all again one day.

That's where the new blogs come in. These are not just static archives but living notebooks where I can and do scribble down something quickly for later use - many of the posts here on POTC have evolved from these kinds of snippets when I've sensed that some general trend can be seen from specific examples.

I'm also aware that different people are interested in different things and not everyone wants to subscribe to a blog where the content is so incredibly randomised. But having a different blog for each topic doesn't suit me either. So here's how things stand :

First and foremost is this one : a place to write finished pieces, but also now with an increased effort on concision and, I hope, more silliness again.

Second, there's Little Physicists. That one focuses heavily on astronomy and data visualisation - stuff I can comment on professionally and quick, rough-and-ready data visualisation tests.

Thirdly, the most active blog is Decoherency, which is basically a shorter version of this blog : everything else I'm interested in, but with much shorter and more focused posts. Lots of things about the latest political developments, rational thinking, nature, and whatever else I happen to find interesting.

These two new blogs are already active, so check them out, follow them if you like, and reshare anything you find interesting because it sure as hell won't reshare itself. Both still have "placeholder" posts which need updating and I'll also be going through all three blogs to replace links to Google Plus posts that might soon disappear. It's been a truly enormous amount of work to archive years of material, but it's now basically complete. And if you want to follow me directly on social media, you can find a complete list of my internet locations here.

Saturday, 9 February 2019

Everyone loves Nazis. Well, loves to punch them or shoot them or whatever. As human beings, their sole redeeming feature is their dress sense, which is what makes them perfect pantomime villains. From their dabblings in the occult to their terrifying superweapons, Nazis make ideal movie enemies. Not to mention that they were evil as hell.

So evil that they turned into zombies in Dead Snow. Much more scary than the losers with little garden torches in 'murica.

Today's post is not another rant about free speech or politics. Nope, this one will answer a much simpler question : what would happen if you put Nazis in space ?

You are no doubt thinking that the answer is simple : a movie even stupider than Dead Snow.

You're not wrong. But there's one aspect of the Nazis that raises some genuinely awkward questions for space travel. Yes, you've guessed it - I'm talking about eugenics. While genetically modifying humans on Earth causes no end of uproar, the idea that we should try and adapt ourselves for the purposes of long-duration space travel is somewhat safer (probably because there's no immediate danger of anyone actually trying it).

The idea's been around for ages, with perhaps a certain grim acceptance of its inevitability. After all, space is such a very different environment that modifying ourselves to make it easier to live on other worlds might be legitimately necessary.

That's the plot of Netflix's truly awful The Titan. At least Iron Sky is vaguely entertaining.

But trying to adapt ourselves to lower gravity or different temperatures isn't remotely feasible for now. There is, though, a much more pressing matter than needs to be addressed : how many people does a multi-generational space mission need ? As we all know, there's only so much inbreeding humans can tolerate before they get a bit... Targaryen. On a small spaceship that could be a real problem.

Only with extra toes on each hand and whatnot. It would be bad, is what I'm saying.

Do we really need multi-generational interstellar spaceships at all ? If we keep things within the bounds of current physics and known technologies, then definitely yes. It would take us millennia to reach the nearest star at the speed of our fastest spacecraft to date. There are known ways to speed this up, but none with much of a chance of getting us there within a few generations. So how many people do we need to take along in order for the crew to not inbreed themselves to death ? Can we let people breed with whoever they want, or are we going to need at least some minimal breeding controls ?

Lots of people raise lots of objections at this point. Some suggest alternatives like hibernation, but this is far from proven. Some suggest we just send robotic probes, but these people suck and I don't like them. Others suggest not taking a living crew but just frozen embryos, to be raised by robots upon arrival. Yeah, nice, but you try programming a robot to raise a child and have it not turn out to be a gibbering wreck. Even many human parents can't manage that.

Because that's not creepy in the slightest.

A more sensible approach might be to take a skeleton crew (no, not literally, skeletons are terrible at sex) with just the bare minimum of people and a large fertility bank. That way the population would always have a huge genetic pool to draw on, preventing the need for a gigantic crew to avoid inbreeding. Not a bad idea at all, actually, but not perfect either : the crew would still need strict breeding regulations and embryo (or other genetic material) storage is untested over such a long period. There's never been a population so reliant on artificial insemination. Of course, any multi-generational ship probably would take along as much frozen genetic material as it possibly could, but it still seems like a good idea to establish how small a population could maintain the species the old-fashioned way. We know with certainty that that works.

That's where HERITAGE comes in. This is a code written by my former housemate and prolific author Frédéric Marin. Now, I should warn you that by training Frédéric specialises in studying active galaxies by looking at their X-ray polarisation, which is a littttle bit different from the problems of managing a bunch of horny space Nazis. And I know, I've said many times that being a specialist doesn't mean you get to prattle on about any subject you have a passing interest in. That's true, but :
- It's possible to study multiple fields in great depth
- If you do step outside your specialist field, I never said you should be ignored completely - you're allowed to have hobbies
- If you reach sufficient competency in any field to get into a peer-reviewed journal, you ought to be taken reasonably seriously
- We'd be only too happy to take genuine constructive criticism from people who understand this stuff better than we do
- GOD DAMMIT IT'S A PAPER ABOUT SPACE NAZIS.

Let's simulate the space Nazis then !

HERITAGE is a code designed to simulate the survival of multi-generational crews. It's an agent-based approach, so it has little "objects" representing each crew member. Each one has various attributes, such as gender, age, fertility, and genetic history. That is, the code can track the family tree of each crew member and calculates the degree of inbreeding over time (assuming that the initial generation are all completely unrelated). This then affects the probability of infertility. If too many couples fail to reproduce, then eventually the crew will die off completely. The spaceship itself isn't modelled, except to impose a fixed, arbitrary population cap - a population higher than that is deemed to have failed. What we want to try and get is a nice, steady, small population.

The parameters of the would-be colonists are somewhat random, and this can have important consequences. For example a woman with a high infertility may not produce any children in one simulation, but one with the same level may still reproduce in another. And that affects the next generation in an unpredictable way. So the code runs the mission setups multiple times - 100, to be exact. This accounts for the random variations and gives far more reliable statistics : for example, sometimes missions fail and sometimes they succeed with the exact same starting conditions, so we can quantify the probability of success.

Paper I in this series was a basic introduction to the code presenting some comparisons to previous works. There have been previous claims for the population requirements, though surprisingly few which try to numerically model it. Those that do are in stark disagreement : one favours a few hundred and the other a few thousand or even more.

HERITAGE can be used to set up the initial generation in accordance with the parameters given in the earlier studies. As a first step though, Frédéric decided to just let the crew really go at it, with no constraints on breeding whatsoever. In this promising setup for a space-based orgy that would put the most depraved pornography to shame, the crew don't give a damn about such trivialities as diminishing resources or incest or anything boring like that. Pretty much all they do is make babies.

Which is quite difficult in a space suit.

As you might expect, this worst-case scenario is Bloody Stupid and doesn't work. The initial population of 150 grows rapidly, reaching 5,000 after 200 years* (and that's including a random disaster which wipes out 30% of them early on in the simulation). God knows what it would be after a few millennia but it would be a lot.

* All missions in paper I last 200 years, for consistency with the previous works.

The other scenarios fared a bit better. Both of these imposed some basic, very similar breeding restrictions, e.g. a maximum number of children per woman and/or an allowed age range for procreation*. Both give very similar results in terms of the crew numbers over time : there are still people on board, but they're not terribly healthy or numerous. The smaller crew is pretty much doomed - the amount of inbreeding is rising steadily at the end of the simulation while their numbers are declining. The larger population fares much better, especially in terms of inbreeding, but the crew numbers are still in a worrying decline.

* Which at the rather high end of 35-40 years old means the crew is filled with space milfs.

I seriously wouldn't if I were you. This settlement founded by a handful of mutineers from the Bounty in1790 is not far off extinction.

Interestingly, none of these scenarios impose any genetic rules on who can breed with who. Rather they only manipulate the initial age distribution of the population, the age at which people are allowed to breed, and the number of children allowed per person. By keeping these ranges narrow, the age of each generation remains very similar. So there might be half the crew being all age around 50 while the other half are all age 25, say, with no-one in between. The motivation behind this was to keep the population from rising exponentially whilst maintaining the maximum genetic diversity available to each generation. That's not how I would have done it, but that's what the other people did.

What this showed was that yes, you could probably manage with a big enough crew, but keeping the breeding rules fixed for all time was a silly idea. Sometimes the crew numbers are quite close to the population cap, so less babies are required. At other times the population numbers are smaller, so more people need to get busy. Limiting the age range at which people can breed is the wrong way to go, as is insisting on a two child per couple rule. As with all things, rules have to take account of varying circumstances.

That's not very Nazi. Be more Nazi.
While paper I tested the code against other people's claims, paper II had a more sophisticated goal : compute the minimum number of people needed for a sustainable population. This time genetics was allowed to determine who could get jiggy with who. And it used a much longer duration for the simulation : 6,300 years, the length of time it would take to reach Alpha Centauri at the fastest speed currently possible. We're talking about a span of time far longer than the ancient empires of Rome and Egypt combined, which is no mean feat. In fact, with fixed breeding rules even a population of 14,000 people isn't enough to survive this long. So we definitely need to vary things !

The first tests didn't have any genetic controls, but just varied the permitted number of children. If the population reaches 90% of the ship's capacity, breeding is prohibited (save for anyone who got pregnant while the population was smaller). When the population falls again, the maximum number of children permitted per couple is 3 so the population can grow again, unlike in the earlier tests when it was only 2. Sounds sensible, right ?

It is, but it doesn't work. The breeding age was initially kept to the same narrow windows as in the previous tests. That's fine at first. But as there are random variations in fertility and breeding ages, over time the age of the different generations starts to slowly disperse : there are plenty of healthy, sexy people on board, but there are fewer and fewer within the narrow permitted breeding age range. With the standard age window of 35-40, the ship dies in a thousand years. An impressive survival time to be sure, but an unnecessary extinction. With 34-40 they make it for 3,000 years. But at 33-40 they surpass a critical threshold and reach a population which is stable indefinitely.

The big drop at ~2,500 years is a random disaster. The recovery afterwards shows that the breeding rules allow a truly stable population to survive for very long periods.

So in fact here it seems that more liberal policies on procreation are what's needed, not more fascist ones. Of course those rules still have to be enforced though, because no rules at all leads to utter disaster, but they're less restrictive than the previous cases.

The problem is that the crew aren't perfectly healthy. They show inbreeding levels, on average equivalent to banging one's first cousin (the code doesn't account for what people would naturally choose to avoid and just pairs up people randomly, so on occasion people go full Lannister). It's not fatal to the health of the crew, but it would probably nice to avoid.

Time to get just a little bit Nazi.

Or as the paper says, "For a purely
genetic safety purpose, we will then restrict inbreeding within
crew members in the following section." And that's why we have outreach articles.

The code can not only monitor inbreeding, but also actively prevent crew members from reproducing with others who are are too closely related. The more strict the inbreeding, the greater the number of missions which fail because there are too few available breeding partners.

The inherent randomness of the process is very important here. Missions either fail or succeed within the first few centuries. Those where the population reaches the ship's arbitrary capacity (500) inevitably succeed and their population remains stable forever after : the population is so large that inbreeding is relatively easy to avoid. Those which don't are driven to extinction. One particularly nerve-wracking example shows a population which reaches 200, perilously falls back to a mere 100 but then makes a spectacular recovery into a perpetually stable 500.

So the obvious question is : what's the minimum starting population we can get away with ? We can do that by keeping the same (population-dependent) breeding rules and systematically testing each scenario, running it multiple times to determine the success rate. Here's the result :

This is for the case of completely preventing all inbreeding. For a population of less than 32, this is absolutely impossible and the missions always fail. Above this, some missions do succeed. But to be really sure of success you need at least 98 : above this, all missions reach their destination successfully with no inbreeding.

If you were really careful, you could probably get away with less than 98 even if you allowed some inbreeding. The code accounts for inbreeding affecting fertility but not much else : it doesn't do anything really sophisticated like, say, having certain genetic strains be able to counter the negative effects of inbreeding in one's ancestors. And the paper established the minimum starting number, it doesn't actually say the minimum sustainable number - the successful missions are ones in which the population always grows to the permitted maximum. So the sustainable population level at which inbreeding can be prevented is certainly somewhere between 100 and 500, but we don't know the exact value. Still, we can say that you don't need thousands or tens of thousands : you need a small village, not a town.

And you don't need to be that much of a Nazi. You need some breeding rules, but in the end not that much worse than people tend to accept naturally anyway. Sure, you can't get it on with your hot, sexy brother, but if you want to do that you've got deep psychological problems anyway. The most prohibitive restriction is the number of children, but whether people are likely to want to raise huge hordes of screaming brats on a spaceship is quite another matter.

Sorry Hitler, but this spaceship is all about maximising diversity.

Cows in space ?

Popular documentary series Astro Farm explored the issue of farming in space in some detail. For paper III, we didn't even attempt to compete with that level of quality. Apart from some refinements to the code that use more accurate fertility data (which turns out not to affect the main result), this paper looked at how much food we'd need to keep the 500-strong population of Nazi space milfs happy and healthy.

The easiest way would be to feed the crew nothing but sweet potatoes. This crop produces the highest amount of edible energy per farmed area, so this is what you want to keep the required area as small as possible. The code was modified to include the height and weight of each crew member (which vary over time according to medical data), as well as different activity levels which determine how much energy they need. Using only sweet potatoes and aeroponics (the most efficient farming method), a farming area of 0.012 km2 is required - an area about 110 m on a side.

One can only imagine what happens when you take a bunch of pseudo-reverse-Nazis, whack 'em on a spaceship and send them hurtling into the void for 6,300 years with nothing to eat but sweet potatoes. It would probably be better to give them a more varied diet, so we did this using recommended dietary requirements. And yes, that includes cows for milk production.

That raises the required area to about 0.5 km2 - an area 700 metres on a side. Even using conventional farming this only doubles to 1 km2 because most of the area is for the animals rather than crops, and you can't grow cows aeroponically. At least, that procedure is certainly not recommended.

Also, the area allowed per animal was based on data for decent living conditions, not battery farming. Happy space cows FTW !

These numbers seem pretty small, so we did some sanity checks from real-world data. These don't have the benefit of aeroponics or perfect climate control or year-round harvests, yet the numbers are still only a few times what we get. So we're likely in the correct ballpark. Our small but intrepid crew don't need vast areas of farmland to sustain themselves.

Some quick, crude estimates find that in a ship rotating to give Earth-level equivalent gravity, we'd need a cylinder 224 m in radius and 320 m long. But if we allow multiple floors then things get smaller. In a rotating spaceship each floor will have a slightly different level of gravity, getting weaker and weaker towards the centre of the cylinder. If we allow farms on floors to gravity as low as 0.9 g (1 g = Earth gravity, 9.8m/s2), then the ship could be just 106 m long. Or at 0.5 g it would be just 25 m long. We could probably even reduce this more, since plants probably don't care too much about gravity. It's a big structure, to be fair... a small skyscraper in space. But it's not anything remotely like as ambitious as the gargantuan O'Neil cylinders proposed back in the 1970's.

Such a ship would be nice and all, but massive overkill.

Summary
What you need for a minimum viable interstellar mission is about a hundred people who don't mind the government intervening in the bedroom and a pretty large spaceship. What we haven't yet established is the really important figure : the minimum mass. Mass dominates everything in space exploration because of the ferocious cost of launching anything out of Earth's deep gravity well. Even with the reductions coming from resusable rockets, the cost isn't like to fall to the point where we could even contemplate an interstellar mission anytime soon.

And let's not kid ourselves : there's an awful lot more work to be done to establish a realistic minimum mass. We need to account for shielding to protect the crew from cosmic rays, for a plausible engine design and the mass of propellant (or some other technique) needed to get the ship to a useful velocity. We need habitation space for the crew, unless we want them to share the stables with the cows. We need water, which is likely to be the topic of the next paper. And we need to account for how efficiently biowaste can be recycled, otherwise our farms would be no better than taking on a whole load of stored food. We made the implicit assumption here that waste can be recyled with 100% efficiency, so the mass of the starting crops is all that's needed. The opposite extreme case of zero recycling demands a food mass of many millions of tonnes, which would be extremely annoying.

More difficult, perhaps, would be how to ensure the crew don't go stark raving mad after six millennia floating in their tin can. No-one has ever designed a society able to manage that level of stability before. With such a small population, warfare is simply intolerable - the risk of complete extinction too great. How the hell we could manage that, I don't know.

But to end with a positive note, we've shown that even with near-term technologies, an interstellar mission isn't totally outlandish. It is, based on what we know so far, at least possible. That is only a first step, but an extremely important and necessary one. For a project like this, one has to accept that we can't tackle all the issues at once. Instead we have to chip away at the whole massive, complex edifice bit by bit. Our ship full of pseudo-reverse-Nazi-milf-space farmers is just the start.

Though the next step probably won't be to add dinosaurs, unless anyone out there can think of a good excuse... ?

Thursday, 7 February 2019

At 25 pages in length, with 30 equations and 23 figures, you could be forgiven for wondering what exactly it is about this paper that justifies the title : "Ram Pressure Stripping Made Easy". I mean, tying shoelaces is easy. Getting drunk is easy. Solving 30 equations is definitely not easy.

Well, I shall tell you. The reason is that we get periodic deliveries of catalogues from Edmund Optics. No-one seems to have the foggiest idea why we get them, but we do anyway. And one of these literary marvels featured the slogan, "Imaging Made Easy" (it was an advert for a camera). Having already rejected, "Ram Pressure Stripping For Dummies" as a title on the grounds that no editor would probably let that pass, and we'd probably be sued for trademark infringement if it did, "Ram pressure stripping made easy" was quickly adopted as an ideal substitute.

Okay, fine. So what's the paper actually about ? Yes, it's time for another, "Rhys explains a paper he worked on in the silliest way possible" post.

(I should note that I'm only the third author on this one, so as not to take credit for the first author's truly immense (many years !) amount of work on this.)

Ram Pressure Stripping : Nothing To Do With Naked Sheep

If you follow this blog much, you'll know that I spend a lot of time investigating galaxy interactions. Galaxies are pretty hefty things, so encounters between them can do a lot of damage. Stars and gas get strewn about all over the place - the "bloody entrails of galaxy interactions", as Robert Minchin once put it.

But the most dangerous thing to a galaxy isn't necessarily another galaxy. Under certain conditions, galaxy-galaxy interactions can be devastating. You might think that this would be when galaxies collide head-on or at terrifying speeds, so the more galaxies and the faster they move, the more dangerous things get. You'd be wrong.

It turns out the most hazardous environment for galaxies are small groups, where interactions occur most slowly. The thing is, galaxies aren't cars. The processes at work have nothing to do with tensile strength or whether the drivers made eye contact beforehand. Instead, galaxies are completely at the mercy of gravity. And what you need for gravity to do the most damage - to perturb the stars and gas most strongly and whack 'em out of their nice stable orbits - is acceleration... and all that takes is mass and time. The longer the encounter lasts, the greater the induced acceleration, and the worse the destruction. So the slower galaxies are moving, the longer and more devastating each encounter is.

Sticking with the disgusting metaphors, tidal encounters are a bit like galactic torture. The higher the force the worse things get, but what you really need is a continuous force over time.

In a galaxy cluster things are different. There there can be hundreds or thousands of galaxies, and the huge mass of the cluster means they're all moving very, very fast (typically thousands of times faster than the proverbial speeding bullet). It's certainly chaotic, but it's so chaotic that, like a movie so-bad-it's-good, it's not actually that awful for the galaxies. They have lots of encounters with other galaxies, but they're all over so blindingly fast that there's not much time for any damage to be done. And because they're all in random directions, whatever minor disturbance they do cause tends to be balanced out. Only very rarely are there any seriously damaging collisions; most of the time the encounters don't remove more than 5-10% of the gas (even after many billions of years of repeat encounters) and even less stars.

For dwarf galaxies things are a bit worse, but for massive galaxies at least, clusters are like swarms of lazy, passive bees that just want to buzz around all day and never sting anyone.

Unfortunately, clusters are more than just a giant swarm of galaxies moving at the kind of speeds normally associated with a teenager's wrist. They also contain their own gas : really hot, incredibly thin (insert offensive supermodel joke here), but filling the entire cluster. All galaxies that move through this intracluster medium (ICM) push on this gas as they move through it, exerting a ram pressure as they push it out of the way.

Some galaxies - the really crappy discount supermarket ones that nobody likes - don't have their own gas. They're not much affected by this ram pressure : the ICM can simply flow between the stars pretty much unaffected. But others do have their own gas, and that causes problems. Since it fills the whole volume of a galaxy, this gas has to interact with the ICM. If the galaxy moves slowly enough, then the ICM can basically flow around the galaxy and not cause too many problems. If it goes too fast though, then the ram pressure can become so high it can push the galaxy's gas right out into the intergalactic void where it's lost forever.

Stars take up a miniscule fraction of a galaxy's volume. So when a galaxy consisting of just stars encounters some external gas (left), it can flow right through it unimpeded. That's not possible for galaxies which do have their own gas (right) since the gas fills the entire galaxy : the external gas inevitably collides, causing a build-up of pressure.

So in clusters, galaxies have nothing much to fear from each other because they're moving so fast. Speed may save them from each other, but that's also exactly what makes the ICM so dangerous, able to strip out the entire gas content of a galaxy in a single orbit. And without gas, star formation completely shuts down. Ram pressure stripping is probably the most important mechanism driving galaxy evolution in clusters.

Let's simulate it then !
We can certainly do that, yes. We can numerically simulate galaxies falling through clusters and see how the two different gas components interact with each other. Here's one I made earlier :

This is a particularly smooth example. Usually much more turbulencedevelops in the stripped gas wake.

This isn't exactly fast though. Setting up simulations like these is a laborious and time-consuming procedure to say the least. Wouldn't it be nice if we had some simple equations to tell us the main results ? Like, how much gas is gonna be lost given a certain amount of ram pressure acting for a certain time. Or what the size of the surviving gas disc would be after a particular bout of pressure, or how much gas would be displaced but eventually fall back to the galaxy due to gravity.

The very basics of this have already been done, in a classic 1972 paper by Gunn & Gott. It doesn't really say all that much though, apart from how strong the pressure needs to be and at what radius in the galaxy the gas will be stripped. That's nice, but the first author wanted more. He reckoned that you could get a lot more information from the basic equations without having to do all the horrible business of running computationally demanding simulations.

Regular readers might be expecting that I'll now begin a highly detailed breakdown of exactly what the paper says. Well for once, I won't. For one thing it's highly technical and I don't claim to understand every detail, and for another I'm trying very hard to write shorter blog posts. So I'm going to bottom line it.

What did we find ?

Famous poster image from the VIVA survey, showing that gasdiscs get smaller closer to the cluster centre.

First, the effect of the ram pressure depends on how quickly it lasts. Stars in the galaxy don't just orbit around the centre, but also move up and down slightly as well. The effect of the ram pressure turns out to be quite different depending on whether the pressure "pulse" lasts for more or less time than this period of oscillation.

Second, the equations allow us to calculate the deficiency of a galaxy - how much gas it's lost in comparison to a similar undisturbed galaxy - given some basic parameters (total mass, rotation speed, that sort of thing). This makes it easy to compare with observations, since often we can only measure the mass of gas and not its radius.

Third, we can examine how much gas the ram pressure would truly strip and push off into deep intergalactic space and how much would just be temporarily displaced, only to fall back to the galaxy after a short time. Some observed streams in galaxies might not indicate true gas stripping but only this temporary displacement.

Fourth, we can make surprisingly detailed comparisons with observations. Knowing where a galaxy is in a cluster, and knowing how dense the ICM is that that point, we can predict how much ram pressure we expect it to experience at that point. And given its deficiency measurement, we can also get the pressure needed to remove that much gas. So we can work out whether we expect a galaxy to be currently stripping, not stripping, or if it was stripping in the past. And by making some assumptions about the orbit, we can even estimate for low long it's been losing gas.

That sounds neat ! Can I have a go ?
Yes you can ! If you were worried about having to do nasty things to equations, worry no more. Probably the most useful result of the paper is not the underlying equations, but the online, interactive simulations that allow anyone to use them without doing any calculations. The first author is a master of javascript (his whole webpage is well worth checking out). I am not, but I did at least persuade him that the tools he routinely developed to play with the results would be worth including in the paper.

What I really like about these is that they can be run in a few seconds (or maybe a few minutes at most), in an ordinary web browser with no installation procedure needed. Compared to other ram pressure simulations, which take hours, days or even much longer, their sheer convenience is so high it's practically erotic. They are much, much better for playing around and getting an intuitive feel for what's happening than any conventional ram pressure simulation. And though there are cases where they won't be entirely accurate and the much more complex models are needed, as long as you don't push them too far they give results which are really rather nice.

GalaxyObserved : Not really a simulation, but a way to help visualise what a stripped gas tail would look like in real observational data given a specified viewing angle

Dark matter in galaxy clusters : simple but extremely helpful little plotting tool to show the different distributions of dark matter and gas in clusters. The y-axis doesn't just have to be density - it can also be total enclosed mass, escape speed or escape time, for example. The kind of parameters you* often find yourself thinking, "hmm, that'd be nice to have, but it's hard to calculate, so oh well."

* Well I do at any rate.

Interactive ram pressure stripping :watch a galaxy get stripped in your browser ! Runs a simple test particle model with ram pressure right before your eyes. You can enter the parameters of the galaxy, the gas it encounters, how long the ram pressure pulse should last, etc. By default it plots the galaxy face-on, colouring the particles to indicate if they're still in the disc, displaced, or fully stripped. But you can use it to plot a whole bunch of other things, including what the galaxy's hydrogen spectrum would look like. Check it out ! The defaults give a nice result.

Looks complicated ? Well it is, to be honest. But it's remarkably easy to use and I guarantee it's at least 97 billion times easier than running a full hydrodynamic simulation.

So that's it : ram pressure made as easy as humanly possible. How far can we take this ? Well, in a future post I'll look at what happened when we did something rather old-fashioned : we made a prediction and tested it. Science may be more complicated than this traditional approach, but dammit it's satisfying to do once in a while. For now, let me just say that these models allow us to establish parameters for large numbers of galaxies that it just wouldn't be feasible to get using more conventional simulations. Although we must always be cautious, they point to a very interesting solution to a problem that's had me scratching my head for several years. But you'll just have to wait and see what that is.

Saturday, 2 February 2019

So... it's been quite a while since the last AAAAAAAA post. Okay, fine, it's been six months, when nominally these posts were supposed to be weekly. Oops. Sorry about that. The problem is that updating the full Q&A page is quite a laborious task, since blogger only allows internal page links if you edit the HTML and doesn't give you a method to do this via the WYSIWYG editor. Also, with the demise of Google Plus - which is currently barely functional and will be gone completely come April - there just haven't been that many questions to answer. Not to mention various other ongoing projects that have demanded my attention.

But though the upcoming doom of Google Plus is a travesty of a farce of an injustice (more on that in a future post), as far as the AAAAAAAA posts go there is good news : I am now active on Quora. Quora is a huge Q&A site where people can ask questions about anything they like. It's also social media of a sort, in that you can follow people and like/dislike content. It has a big advantage of ready access to an endless series of questions and a very easy-to-use interface. So if you have any burning questions I highly recommend trying it out ! Of course, you can also ask me directly in the comments here.

I'll have more to say on where you can find me on the internet in a subsequent post. Suffice for now that this blog isn't going anywhere, but I'll probably stop trying to collate questions on anything like a weekly basis - at least for the foreseeable future. Once a month is more realistic, but expect bigger collections thanks to Quora's much higher activity rate.

Anyway, here's the latest batch of questions. Don't forget to click on the links for the longer and less sarcastic answers !

They don't yet have a SPACE DRAGONS Because Space Dragons T-shirt, but if they do, I'm buying one and you can't stop me. "Rhys, what are you babbling about ?" I hear you ask. Well, I shall tell you.

Towards the end of 2018 I was asked by the lovely people at the SOFIA observatory to have a bash at visualising some nice new data they'd collected. SOFIA is not a Bulgarian telescope that's decided to SHOUT at people, but an acronym for the Stratospheric Observatory For Infrared Astronomy. That's much cooler than it sounds, because what it actually is is a 2.5m telescope that flies in the back of a specially-modified NASA jumbo jet.

With the windows open. They don't usually let you do that on a commercial flight.

Of course, flying the telescope up as high as you can is better because it gets it closer to the stars... no, not really. The actual reason is that it avoids emission from much of the atmosphere, which at certain frequencies makes it very difficult to see anything. Like emission from ionised carbon (CII), for example, which is great for studying star formation.

Since I specialise in gas that isn't forming stars at all, I won't try and bluff my way into making this a science post. No, here my only role was to make pretty pictures. And gas is very, very pretty, after all. Especially in the infamous Orion nebula.

A standard view of the Orion Nebula I grabbed off the internet at random.

Unless you're truly spectacularly ignorant, you've seen images like this before. It's a big, bright target popular with amateur astronomers and surely one of the most photographed things ever, right up there with Megan Markle and her feckless adopted family. So isn't sending up a great big telescope in a jumbo jet to take even more photographs of it a bit like... well, you can imagine Frankie Boyle's inevitable paparazzi joke here for yourself.

Sorry Megan. I'll be good.

As it happens, no. Because this telescope can see it at frequencies never studied before, and, just like with radio astronomy, it doesn't just make 2D photographs but 3D data cubes. Which, as regular readers know, is My Thing. Spectral imaging lets us map not the true 3D structure of the gas, but how it's moving. Which can tell you all kinds of fun things about where it's going, but again, not gonna try and bluff my way through the specifics of this one (I haven't even read the paper). And I'm going to gloss over the details of the visualisation techniques in this post - you can read all about the details here and here if you're interested. If you're not, don't.

Anyway, the data set was rather large (over 700 million pixels) which is pushing the hardware limits, at least in terms of quick tests that are so important for playing around to get things looking fancy. So I started off by trimming the data to make it more manageable. And then, according to a press release :

“When we first saw it, we were standing around my computer screen looking at it and say, ‘Hey, doesn’t that look like a dragon?’ And everybody said, ‘Yeah, that looks like a dragon,’ ” Joan Schmelz, the SOFIA project’s associate director for science and public outreach at the Universities Space Research Association, said today at the American Astronomical Society’s winter meeting in Seattle.

I don't remember who first said it looked like a dragon. It might have been me. It might have been Robert Minchin. It might have been someone else over at the SOFIA site in California. In any case, by trimming the data at just the right point, it definitely looks like a dragon.

"As we rotated the data cube, we got our first glimpse of the structure that we've nicknamed Orion's Dragon," said Rhys Taylor, a scientist at the Astronomical Institute of the Czech Academy of Sciences and a consultant to the SOFIA team, in a press release. "A few people have said it looks like a sea horse or a pterodactyl, but it looks like a dragon to me."

Here's a dirty secret : we never actually say the things they say we say in press releases. They aren't done by interviews with some trilby-wearing journalist scribbling quotable quotes into a notebook. What actually happens is that some unlucky scientist writes it, checks if it's OK with everyone else, then sends it to a press officer for distribution.

If you can't see the resemblance to a dragon, WELL YOU'RE WRONG. Look, here's objectively the best flag ever :

Learning to draw the flag when you're five is much harder in Wales than in other countries.

And here's the Orion data, suitably rotated with some colour adjustments (we'll get to that in a moment) :

Y Ddraig Orion.

Clearly it's a motherfrakkin' dragon. And a bad-ass one at that, in a nebula a thousand light years away seen through a telescope flying on a jumbo jet.

Except, though, that the data here has been trimmed. I never intended to keep using the smaller data set, it was just a way to make playing around with the colour scheme faster and easier. 3D rendering can be slower than a tranquillised snail solving a Sudoku puzzle, so the need to speed things up is actually quite important. I planned to figure out nice settings for the 3D display with the smaller data and then use these to render the final version with only minimal further adjustments needed. That's exactly what happened, but the final movies never made it into the press releases - they just don't look dragon-y enough, I suppose. Here's the GIF you may have already seen :

But here's something you haven't seen : the full data set, rendered with some more complex processing :

Without the top cut off it looks distinctly less dragon-like ! We went through a whole bunch of iterations on this. The data was smoothed slightly, which reduces the noise but also the detail. The colour scheme is much more complex, which makes things generally prettier but makes it a bit harder to interpret (there isn't a nice direct relation between colour and intensity here).

Strictly speaking the data isn't truly three-dimensional in the normal sense, though it can be helpful to visualise it like this. What the data actually is is a series of images each at slightly different frequencies, equivalent to different velocities depending on how the gas is moving. Instead of layering them together to make a cube, we can also use these adjacent frequency "channels" to give different colours. Again, you may have seen a trimmed version of this, but here's the full data set :

As we step through the cube, adjacent channels are used to give the red, blue and green components. Of course the colours are false, but then, our eyes can't see at these frequencies anyway, and the structures you can see are absolutely real. It looks lovely, but we can do even more. The SOFIA website includes a 3D movie you can view with those traditional red-blue glasses, which I'm pleased to discover was shown at the AAS meeting...

... but you, dear readers, deserve better. So to finish off, here's a 360, 3D VR flythrough. You'll need a VR headset to enjoy this in its full glory, but YouTube should allow you to enjoy the 360 content in a regular browser and even anaglyph (red-blue) 3D if you're so inclined.

So there you have it : a gigantic space dragon seen in 3D through a telescope sticking out the back of a jumbo jet. If that's not at least slightly cool, I'm going to have to have spiritual breakdown and take up Tibetan yak farming or something.